Microstructure and stress gradients in TiW thin films characterized by 40 nm X-ray diffraction and transmission electron microscopy

Abstract

The functionality of TiW diffusion barrier is often correlated with the presence of impurity atoms, diffusion of neighbor layer atoms, and depletion of titanium from the film and seldom attributed to the microstructure of the film. Here in this work, morphological aspects of sputter deposited TiW thin films as well as the influence of process parameters on residual stress of the film were investigated to understand their role in the functionality of TiW as a barrier. Two films, one tensile (+2 GPa) and another with compressive stress (−2 GPa), deposited by varying process conditions show distinctive differences in their microstructure. Both films were composed of α–W crystal structure but film with compressive stress has densely packed microstructure with no inter-granular porosity unlike to the tensile stressed film, where substantial porosity was present. Furthermore, TiW films were studied using a cross-sectional synchrotron 40 nm X-ray nano-diffraction technique which revealed that a gradual stress relaxation takes place in tensile stressed TiW film during its growth while in the compressive film no such relaxation was observed. The stress relaxation is mainly attributed to the increase in grain size during film growth of the tensile film. Based on these investigations, a growth model was put forth to correlate the deposition conditions and morphology of the TiW films grown.